A number of studies have suggested that drinking water hardness is linked somehow to deaths from arteriosclerotic heart disease (ASHD), with increased deaths in areas with soft water. It has also been suggested that some soft waters dissolve trace metals from certain types of pipe, and that these trace metals, notably cadmium, may be responsible for the relationship of soft water to ASHD. It seems very important to try to establish whether or not such a relationship exists. If it does, relatively simple procedures at water treatmnt plants could prevent many deaths from ASHD. Washington County, Maryland has a wide range of hardness in its drinking water sources. Using information already available, it is possible to classify each of the 91 enumeration districts in the county with respect to mean hardness of water. Information on the type of water pipes in each house and the use of water softeners will be collected during the summer of 1975. The base population for the study consists of persons enumerated in the private census of 1963. Cases of ASHD will be persons in the census dying from ASHD in the subsequent 12 years. The population can be corrected for losses because of a sample investigated in 1971. Death rates from ASHD will be calculated for various levels of water hardness with adjustments for the effects of cigarette smoking and socio-economic factors. If a significant association of ASHD with water hardness is found, the additional association with type of water piping will be investigated. Because the proposed study is based on characteristics of water as received by the consumer, it is free of some major disadvantages of studies based on area correlations between death rates and water characteristics at the treatment plant.